The present invention relates to recording medium with a carbon-containing overcoat, particularly, a carbon-containing overcoat with a variable carbon density, and a method of making the same.
Most modern information storage systems depend on magnetic recording due to its reliability, low cost, and high storage capacity. The primary elements of a magnetic recording system are the magnetic recording medium, and the read/write head. Magnetic discs with magnetizable media are used for data storage in almost all computer systems.
FIG. 1 shows the schematic arrangement of a magnetic disc drive 10 using a rotary actuator. A disc or medium 11 is mounted on a spindle 12 and rotated at a predetermined speed. The rotary actuator comprises an arm 15 to which is coupled a suspension 14. A magnetic head 13 is mounted at the distal end of the suspension 14. The magnetic head 13 is brought into contact with the recording/reproduction surface of the disc 11. The rotary actuator could have several suspensions and multiple magnetic heads to allow for simultaneous recording and reproduction on and from both surfaces of each medium. A voice coil motor 19 as a kind of linear motor is provided to the other end of the arm 15. The arm 15 is swingably supported by ball bearings (not shown) provided at the upper and lower portions of a pivot portion 17.
A conventional longitudinal recording disc medium is depicted in FIG. 2 and typically comprises a non-magnetic substrate 20 having sequentially deposited on each side thereof an underlayer 21, 21xe2x80x2, such as chromium (Cr) or Cr-alloy, a magnetic layer 22, 22xe2x80x2, typically comprising a cobalt (Co)-base alloy, and a protective overcoat 23, 23xe2x80x2, typically containing carbon. Conventional practices also comprise bonding a lubricant topcoat 24, 24xe2x80x2 to the protective overcoat. Underlayer 21, 21xe2x80x2, magnetic layer 22, 22xe2x80x2, and protective overcoat 23, 23xe2x80x2, are typically deposited by sputtering techniques. The Co-base alloy magnetic layer deposited by conventional techniques normally comprises polycrystallites epitaxially grown on the polycrystal Cr or Cr-alloy underlayer.
A conventional longitudinal recording disc medium is prepared by depositing multiple layers of metal films to make a composite film. In sequential order, the multiple layers typically comprise a non-magnetic substrate, a seedlayer, one or more underlayers, a magnetic layer, and a protective carbon layer. Generally, a polycrystalline epitaxially grown cobalt-chromium (COCr) magnetic layer is deposited on a chromium or chromium-alloy underlayer.
The seed layer, underlayer, and magnetic layer are conventionally sequentially sputter deposited on the substrate in an inert gas atmosphere, such as an atmosphere of pure argon. A conventional carbon-containing overcoat is typically deposited in argon with nitrogen, hydrogen or ethylene. Conventional lubricant topcoats are typically about 20 xc3x85 thick.
There is a demand in computer hard drive industry to develop recording media with high areal storage density. However, an increase in high areal density often demands that the flying height between the read-write head and the recording media have to be minimized. Current magnetic hard disc drives operate with the read-write heads only a few nanometers above the disc surface and at rather high speeds, typically a few meters per second. Because the read-write heads can contact the disc surface during operation, a protective overcoat is applied to the disc surface to protect the recording media from inadvertent collisions with the recording head. The protective overcoat also protects the recording media from environmental stresses, e.g., moisture or corrosive environments.
As hard disc drive technology is pushed to higher and higher recording densities, the industry is faced with an increasingly difficult task of maintaining the tribological robustness of the head-disc interface (HDI). The durability and reliability of recording media is achieved primarily by the application of two protective layers: carbon-containing overcoat and liquid lubricant film. Diamond-like carbon (DLC) has been used as a protective layer for magnetic recording media. Conventionally, DC or RF magnetron sputtering could be used to deposit a thin layer of DLC on disks.
Manufacturers have used sputtered amorphous carbon-containing overcoats to provide protective coatings for recording medium. U.S. Pat. No. 5,462,784 discloses using a thick fluorinated diamond-like carbon layer of thickness in the range between 30 xc3x85 and 300 xc3x85. U.S. Pat. No. 5,182,132 discloses a diamond-like carbon layer deposited by plasma enhanced chemical vapor deposition (PECVD). European Patent Application No. 595,564 discloses a diamond-like carbon layer containing carbon and hydrogen.
Most disc drives produced currently operate in the Contact Start/Stop (CSS) mode. Since the recording head contacts with recording media during takeoff and landing, corrosion of the magnetic layer due to a large number of CSS cycles could be a major cause of drive failure. To ensure good corrosion resistance, applicants have found that an overcoat having the ability to prevent corrosion of the magnetic layer is required and a traditional overcoat material, such as hydrogenated (a-C:H) or nitrogenated (a-C:N) carbon, could be insufficient in protecting the hard disc media or read-write head from corrosion at the thickness level of less than 5nm.
With the continuous increase of the recording density, the carbon-containing overcoat becomes thinner and thinner, even less than 3nm, at which thickness applicants have found that the conventional sputtered carbon may not be satisfactory as a protective overcoat. To improve the mechanical properties of the carbon-containing overcoat, thinner and harder films, such as plasma enhanced chemical vapor deposition (PECVD), ion beam deposition (IBD), and filtered cathodic arc deposition (FCA) were tried. These techniques deposit a carbon-containing overcoat by employing high carbon ion energy of more than 50 eV, typically, more than 50 to about 150 eV. These techniques could produce a hard and dense carbon film due to the much higher carbon ion energy (50-150 eV) used in the process of sputtering the carbon-containing overcoat.
FIG. 3 shows that the higher the carbon atom energy, the greater is the density of the carbon-containing overcoat. However, the impingement of the energetic carbon ions onto the magnetic layer can cause the mixing of carbon with the magnetic material at the interface of the magnetic layer.
Therefore, there still exists a need for a thin overcoat for a magnetic recording medium that has good corrosion resistance and tribological performance while minimizing the interfacial mixing of carbon and magnetic elements in the magnetic layer of the magnetic recording medium.
An embodiment of this invention is a magnetic recording medium comprising a substrate; a magnetic layer; and a carbon-containing overcoat having a thickness of about 150 xc3x85 or less and comprising a first carbon density and a second carbon density different from the first carbon density, wherein the magnetic recording medium does not contain a dielectric layer between the magnetic layer and the carbon-containing overcoat, and preferably, the carbon-containing overcoat directly contacts the magnetic layer. The first carbon density is about 1.8 g/cm3 or less, the second carbon density is higher than the first carbon density and the difference between the second carbon density and the first carbon density is at least about 0.025 g/cm3. 
In other embodiments, the first carbon density is a density selected from the group consisting of about 1.75 g/cm3 or less, 1.7 g/cm3 or less, 1.65 g/cm3 or less and 1.6 g/cm3 or less; the second carbon density is a density selected from the group consisting of at least 1.8 g/cm3, of at least 1.85 g/cm3, of at least 1.9 g/cm3 and of at least 1.95 g/cm3; and the first carbon density is at a distance of 30 xc3x85 or less from a top surface of the magnetic layer.
In some embodiments, the carbon-containing overcoat comprises a material selected from a group consisting of carbon, hydrogenated carbon, fluorinated carbon, nitrated carbon, amorphous carbon, amorphous hydrogenated carbon, amorphous fluorinated carbon, amorphous nitrated carbon and combinations thereof. The carbon-containing overcoat could comprise at least a first carbon layer and a second carbon layer, wherein the first carbon layer could be deposited using a sputtering process and is closer to the magnetic layer and comprises the first carbon density. Preferably, the second carbon layer comprises the second carbon density, the second density is at least 1.8 g/cm3. 
Another embodiment of this invention is a method of making a magnetic recording medium comprising depositing a magnetic layer on a substrate; depositing a first portion of a carbon-containing overcoat directly on the magnetic layer; and depositing second portion of the carbon-containing overcoat under the following deposition condition:
Yxe2x89xa70.7764 X0.5639
wherein, Y is a thickness of the first portion of the carbon-containing overcoat and X is carbon ion energy per carbon atom in eV during said depositing second portion of the carbon-containing overcoat and the carbon-containing overcoat comprises a first carbon density and a second carbon density different from the first carbon density. The step of depositing a carbon-containing overcoat could comprise increasing the carbon ion energy as the thickness of the carbon-containing overcoat is increased.
In a preferred embodiment, the carbon-containing overcoat has a thickness of about 150 xc3x85 or less on the magnetic layer and the first carbon density is about 1.8 g/cm3 or less. In a particular embodiment, the carbon ion energy is increased from a first range of less than 50 eV to a second range of more than 50 eV during said depositing a carbon-containing overcoat.
The first carbon layer could be deposited using a deposition process selected from the group consisting of plasma-enhanced chemical vapor deposition, ion beam deposition, and filtered cathodic art deposition.
Another embodiment of this invention is a magnetic recording medium, comprising a magnetic layer and means for protecting the magnetic layer.
In this invention, xe2x80x9cmeans for protecting the magnetic layerxe2x80x9d refers to a carbon-containing overcoat of a thickness of about 150 xc3x85 or less directly on the magnetic layer without another layer that is neither a carbon-containing layer or a magnetic layer interposed between the carbon-containing layer and the magnetic layer, wherein the carbon-containing overcoat comprises a first carbon density of about 1.8 g/cm3 or less and a second carbon density different from the first carbon density.
As will be realized, this invention is capable of other and different embodiments, and its details are capable of modifications in various obvious respects, all without departing from this invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not as restrictive.